1. Field of the Invention
This invention relates to optical memories which are adapted to store massive amounts of information and which are operable in a read only mode or in a read, write and erase mode. The invention further relates to a storage element which can be accessed on a selective wavelength basis and includes a multiplicity of memory cells which incorporate cholesteric liquid crystal (CLC) materials.
2. Description of Related Art
For over three decades, magnetic disk and tape media have been the dominant mass storage technologies featuring high capacity and low-cost memories. They have been used extensively in mainframes, personal computers and in entertainment as audio and video tape cassettes. Recently, however, the newer optical mass storage technology has begun to seriously challenge the dominance of magnetic media in mainstream applications. Optical mass storage has the advantage of higher capacity, lower cost per bit, removability and relatively higher immunity from accidental erasures by magnetic fields. Indeed, CD-ROMs, laser disks and magneto-optical drives and media have been introduced to the marketplace and have been accepted by users and are presently enjoying a significant growth rate.
In magneto-optical, M-O, technology, information is stored in a magneto-optical layer by changing the direction of the magnetic domain: "up" and "down" representing electrical "1" and "0", respectively. To write "1", a focused write laser beam heats a spot in the M-O material above its Curie temperature (reducing the magnetization to zero) in the presence of a magnetic field (applied by a coil)in the "up" direction, for example. Upon cooling, the magnetization increases from zero to a final value in the "up" direction. To erase or write "0", the above steps are repeated but the magnetic field is applied in the "down" direction, producing magnetization in the "down" direction. The read operation relies on the M-O Kerr effect which is the ability of the M-O material to rotate, by 3.degree.-5.degree., the polarization of the reading laser beam incident on it. A polarizing beam splitter transmits the incident beam, and upon reflection from the M-O material, directs, toward an analyzer/detector, only the phase shifted (rotated) components of the reflected beam. To read a "1" state, the analyzer is aligned so that it differentially detects the light the polarization of which is phase shifted by the "up" magnetic domain, while it extinguishes the light that is phase shifted by the "down" magnetic domain. Typically, this read operation has good discrimination and a carrier to noise ratio in excess of 50 dB.
This M-O technology is currently enjoying a significant market penetration rate because of its proven higher density than magnetic disk technology and is evolving rapidly to improve its seek time and data rate. This technology faces a wavelength bottleneck that imposes an upper limit on its density, unless a great deal of parallelism is used.
Cholesteric liquid crystals are nematic liquid crystals that have a chiral additive or a side-chain polymer with a polysiloxane backbone. These cholesteric liquid crystal (CLC) materials have cigar shaped molecules which order themselves in an optically active structure in a left-handed or right-handed helix with a helix pitch, P, and have an optical axis which is parallel to the helix axis. CLC materials are described in the following references: S. D. Jacobs et al, Journal of the Optical Society of America, B, VOL. V (XI) pages 1962-1978 (September, 1988); M. Schadt and J. Funschilling, Society of Information Displays, SID XC Digest, Page 324 (1990).
To the extent that in CLC materials are known, no prior art is known which incorporates such materials in an optical memory wherein different layers of such materials are stacked and selectively accessed on a wavelength basis. Also, there is no known prior art which shows how memory cells made of cholesteric liquid crystal materials may be reversibly written into and erased so that such memory cells when incorporated into stacked memory elements may be addressed selectively on a wavelength basis.
An article entitled "Liquid Crystalline Polysiloxanes for Optical Write-Once Storage" by J. Pinsl et al, Journal of Molecular Electronics, VOL. 3, 9-13 (1987) shows a CLC storage region into which a material which converts light to heat has been introduced. Then, in a write-once mode of operation, an irreversible destruction of the helical structure of the CLC material takes place destroying the ability of the CLC region to reflect light completely. To the extent that a write-once mode of operation only is suggested, the present invention may be distinguished over the reference in that it does not suggest the reversible write-erase mode in the presence of electric or magnetic fields as does the present application. This should be clear form the article itself which characterizes the write-once operation as "irreversible". Neither does the reference suggest the stacking of storage elements for selection on a wavelength basis.